In recent years, it has been found that metal fine particles on the nanometer level (hereafter, referred to as metal nanoparticles) may exhibit thermal and magnetic properties different from those of normal metal bulk; and novel reactions and materials utilizing these properties have been intensively developed. For example, although normal gold particles are nonmagnetic, it is known that gold nanoparticles having a size of several nanometers exhibit ferromagnetic spin polarization. In addition, although gold in bulk form is chemically inert, a gold cluster having a size of several nanometers exhibits properties of a Lewis acid; and, on the basis of these properties, coupling reactions and oxidation catalytic reactions due to activation of oxygen molecules are being intensively studied.
Unlike general metal powder particles on the order of millimeters, metal nanoparticles fuse together at a very low temperature with respect to the melting point of the chemical element. For example, silver has a melting point more than 960° C., whereas silver nanoparticles having a particle size of 100 nm or less fuse together even at a temperature of 200° C. or less. Accordingly, when such nanoparticles are produced in the form of ink or coating material, a conductive coating material that allows formation of electronic circuits by printing even on polymeric films having low heat resistance can be provided. For this reason, nanoparticles are a material that is particularly attracting attention.
To cause the low-temperature fusion phenomenon, metal particles need to be produced such that the particle size is minimized. For this purpose, the vacuum evaporation method and liquid-phase methods in which a metal compound in the form of a solution is reduced in the presence of various colloid protective agents have been developed. The latter methods often need a large amount of a colloid protective agent in accordance with the specific surface of the particles to be generated; however, since organic matter remaining on metal nanoparticles can increase the fusion temperature, the amount of the colloid protective agent used needs to be minimized. However, when the amount of the colloid protective agent is decreased, disadvantages tend to be caused: the resultant metal-nanoparticle dispersion becomes unstable and agglomeration occurs to provide coarse particles or precipitation occurs during storage.
It is known that, for example, in the case of forming a metal coating film from a dispersion liquid of silver nanoparticles, to achieve low-temperature baking at 200° C. or less, the amount of a colloid protective agent remaining on the particles is desirably controlled to be 5 mass % or less; when this amount exceeds, it is difficult to achieve a low resistivity on the order of 10−5 Ωcm even by baking at 200° C. (for example, refer to Patent Literature 1). Accordingly, selection of a colloid protective agent that can maintain, in a small amount thereof, stable dispersion of metal nanoparticles is important for achieving the low-temperature baking. In spite of such efforts, to achieve a low resistivity on the order of 10−5 Ωcm, in most cases, heating-baking at 150° C. or more after application is still required.
There is a rare exception that a group of National University Corporation Osaka University and Osaka Foundation for Trade and Industry achieved a volume resistivity on the order of 10−5 Ωcm in the following manner: a thin film is formed from a tetradecane dispersion of silver nanoparticles covered with dodecylamine; this film is immersed in a polar solvent and, in this solvent, dodecylamine coordinated to the surfaces of the metal nanoparticles is dissociated; and the resultant film is dried at room temperature (for example, refer to Patent Literature 2). However, such a step of immersing a coated member in a polar solvent for several hours in advance to remove a protective agent through dissolution is not suitable for applications to packaging techniques including formation of conductive wiring and conductive electrodes on coated members (substrates). In addition, when the step of immersion in a polar solvent is performed, adhesion between a substrate and metal nanoparticles is not ensured and separation of the metal nanoparticles from the substrate is unavoidable. Thus, the achievement of a low resistivity by means of drying at room temperature only is merely a local phenomenon and lacks practicality.